Classical gain control circuitry includes such circuitry as a standard Gilbert gain cell. A Gilbert gain cell or multiplier is generally composed of a transistor pair gain stage for generating an output current and a predistorted stage for controlling the transistor pair gain stage. By controlling the ratio of the input currents in the predistorted stage, the output current may be programmably controlled. A Gilbert gain cell will generally include at least two transistor pairs. However, a standard Gilbert gain cell does not provide exponential current gain control which is often desirable and needed.
Various circuit applications require exponential current gain control. For example, exponentially controlled amplifiers, such as transconductance amplifiers, require exponential or nearly exponential current control to provide exponential current gain. Other circuitry, such as low pass filters with tunable boost or gain, may also need exponential current control to provide exponential current gain.
Implementing exponential current gain is equivalent to implementing constant-decibel current gain steps. However, existing methods and circuitry suffer serious drawbacks and problems. Generally, existing methods and circuitry are silicon or "area inefficient," "offset prone," and often inaccurate. Offset prone refers to the added requirement and burden of ensuring that transistor pairs are properly matched and biased for correct circuitry operation. Overall system accuracy suffers if transistor pairs are not properly matched. Overall system reliability may also suffer due to improper transistor pair matching. Additionally, improper transistor pair matching increases overall system noise.
One method for providing exponential or nearly exponential current gain control involves the use of a resistor network in combination with a standard Gilbert gain cell to control the input currents in the predistorted stage. The resistor network is provided with a plurality of resistors that may be programmably selected to produce the desired ratio of input currents in the predistorted stage. This method not only suffers from the problems and disadvantages listed above but also suffers from circuitry design complexity. The circuitry design complexity is primarily caused by the need to design resistor values for every possible or desired combination of programmable inputs to the resistor network that will result in a ratio of input currents that generate the desired output current.
Another method for providing exponential or nearly exponential current gain control involves the use of an exponential current generator in combination with a standard Gilbert gain cell to control the input currents in the predistorted stage. The exponential current generator includes a voltage input that establishes one of the input currents at a fixed value and generates the other input current as an exponential or nearly exponential input current. The exponential current generator is a programmable generator that includes at least one transistor pair that must be matched during fabrication. In addition to the transistor pair in the exponential current generator, the standard Gilbert gain cell includes at least two additional transistor pairs for a total of at least three transistor pairs that must be matched. Therefore, this method also suffers from the disadvantages first mentioned above and is especially offset prone due to the requirement of transistor pair matching.